1. Field of the Invention
The present invention relates to techniques for monitoring and surveying a hydrocarbon reservoir penetrated by a well. More particularly the invention concerns a method, and a device for its implementation, in which at least two electrodes are located in the well, spaced apart from each other along the length of the well and connected to a current source and to means for measuring an electrical parameter, from which a characteristic of the reservoir is deduced. The parameter is the potential or the current and the characteristic of the reservoir is the resistivity of the geological formations forming the reservoir.
2. Related Art
The production of hydrocarbons needs to be controlled and monitored regularly or continuously, in order to determine the cause of any reduction or stoppage in production and in order to attempt to remedy it. Apart from depending on the installed production facility, production also depends on characteristics of the reservoir, not only static characteristics (i.e. porosity) but also dynamic characteristics (intercommunication between the pores, permeability, etc.). An important item of information in this respect is the position in the reservoir of the hydrocarbon/water or hydrocarbon/gas interface.
It is of fundamental importance not only to detect a possible pocket of water or gas but to know its position at any given time, in order to avoid the water reaching the production well.
The electrical resistivity of the ground is used in a known manner as a representative characteristic of the reservoir. The resistivity of hydrocarbons is much higher than the resistivity of the formation water which carries salt (in a ratio of around 100 to 1). The measurement is effected upon drilling the production well, with a logging sonde having electrodes or with an induction sonde, means being provided to allow the resistivity of the strata which is traversed by the well to be determined.
The presence of the casing, needed for production, presents an obstacle to sondes employing electrical measurements. Moreover, most types of sondes for well logging have a depth of investigation of the order of one meter around the production well and thus do not allow the characteristics of reservoirs to be surveyed on a large scale.
In this context the present invention provides a method and a device for surveying, monitoring and making measurements on the reservoir itself, without affecting production, and in particular for enabling the position of the hydrocarbon/water interface to be determined, with a view to optimizing production.
According to an aspect of the invention, there is provided a method of monitoring a fluid reservoir penetrated by at least one well, comprising the following steps:
at least one electrode is fixed in permanent manner in the well and communicating with the surface under conditions for hydraulically isolating the section of the well in which it is located from the rest of the well and for providing electrical coupling between the electrode and the reservoir; PA1 a current is passed through the reservoir; and PA1 an electrical parameter is measured and a characteristic representative of the reservoir is deduced therefrom. PA1 means, including a current source, for passing current through the reservoir; PA1 means for measuring potential or current; and PA1 at least one measuring electrode fixed in a permanent manner in the well in the region of the reservoir and communicating with the surface under conditions hydraulically isolating the section of the well in which it is located from the rest of the well and providing electrical coupling between the electrode and the reservoir.
According to another aspect, there is provided a device including several electrodes disposed on a support adapted to maintain between the electrodes a spacing which is absolutely constant with time and to insulate the electrodes from each other.
In an embodiment of the invention, more particularly applicable to a production well passing through two regions containing different hydrocarbons, the support is formed by a section of rigid metallic casing, with an associated electrically insulating coating.
In another embodiment, more particularly applicable to an uncased well, drilled specifically for measurement purposes and separate from the production well, the support is formed by an elongate member of flexible or semi-rigid, electrically insulating material.
In a further embodiment, the device includes a cable having a plurality of insulated conductors within an outer sheath, each conductor having its end portion exposed to form a respective electrode.
The electrodes are advantageously fixed in the well by cement injected between the electrodes and the wall of the well.
The cement preferably has electrical resistivity in the same order as that of the ground of the reservoir.
In another embodiment of the invention, a plurality of measuring electrodes and a current injection electrode are fixed in the well, in a permanent manner.
Different measurements are effected at at least one intermediate electrode and relative to different electrodes providing current return.
In a variant of the invention, an injection electrode, a return electrode, a reference electrode and one or more intermediate measuring electrode(s) are used, the measuring electrode(s) not being connected to the injection electrode, and the potential difference is measured between the intermediate electrode(s) or between the intermediate electrode(s) and the reference electrode.
In order to follow the movement of the hydrocarbon/water interface, measurements spaced apart in time are made, and the difference between the measurements is calculated in order to determine a parameter representative of the movement of said interface.
The injection electrode or current source preferably generates a continuous current, or a low frequency alternating current.
According to a further aspect of the invention, there is provided a device for monitoring a fluid reservoir penetrated by at least one well, comprising a support on which are fixed longitudinally spaced apart electrodes, the electrodes being adapted to be connected to a current source or electrode and to means for measuring an electrical parameter (potential or current), in such a manner that each electrode can act equally well as a current electrode or as a measuring electrode.
The invention further provides an installation for monitoring a fluid reservoir penetrated by at least one well, comprising: